Patent GB-467105-A (GB)
Abstract: 467,105. Electromagnetic sound recording and reproducing apparatus. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., MURPHY, F. M. G., and TWEED, C. J. F. Dec. 10, 1935, No. 34216. [Class 40 (ii)] A telegraphone recording or reproducing head comprises a polepiece or polar structure carried from a movable carrier member by flat or leaf carrier means, the arrangement being such that in one position of said carrier means the pole face carried thereby is pressed towards the telegraphone tape, the carrier means being movable with said member in such manner that, in another position of said carrier means, said pole face is withdrawn from the tape. The construction shown comprises a box 3 having a back plate 1 and spring-controlled hinged lid 4, the tape (not shown) travelling through the box in the direction of the arrows, Fig. 1, between polar structures 7, 8. The structure 8 cornprises a laminated bar 18, the central lamination 18<a> of which is longer than and projects at one end beyond the ends of the others as shown in Fig. 5. This bar is riveted between the bottom limb 19 of a U-shaped phosphor bronze strip 19, 20, 21 and another limb 23<a> integral with a shaped plate 23, Fig. 6, the limbs 19 and 23<a> being surrounded by the coil 22. The plate 23 is adapted to receive an ivory &c. face plate 26 preferably made in two parts so as to clamp the central lamination 18<a> therebetween, the end of this lamination being substantially flush with steps 26d on the plate 26. The free ends of the limbs 20, 21 are riveted to a mounting block 28 pivoted on a pin 29 secured to a slide plate 12 which is arranged after the manner of a microscope slide relative to the back plate 1 so that the relative stagger of the poles can be adjusted by rotation of the calibrated knob 13. A member 30 having a leg 30<a> controlled by a tensioned spring 31 is also mounted on the pivot 29, this member carrying a springy abutment strip 32 having a forked end 32<a> engaging opposite sides of the limb 20. The strip 32 is adjustable by a screw 33 whilst movement of the block 28 is limited by an adjustable screw 35. The face plate 26 normally presses on the tape with a predetermined pressure but by rotating the parts 28. 30 anticlockwise the polar structure can be withdrawn from the tape. Such rotation is effected by means of a cam 37 having flats 37<a> and rotatable on a pivot 38 by means of a lever 39 provided with a thumb piece 39<a>. The cam engages an extension on the member 30 so that when the cam is rotated anticlockwise the branch 32<a> engages the underside of limb 20 and lifts the polar structure away from the tape. The lower polar structure 7, which is similar in construction to the upper, is carried by a U-shaped spring from the pivoted member 9 upon which the cam 37 also operates. According to modifications, the lower polar structure may be fixed or omitted altogether, whilst the strip 32 may also be omitted. An operating winding may be provided on each pole or, as described in Specification 439,179, there may be an operating winding on one pole only or an operating winding on one pole and an auxiliary or " shaping " winding on the other. Again, one half of the head may be used for A.C. excitation and the other half for D.C. polarization or, in the case of a recording head, a winding on the lower pole piece may be dispensed with. In some cases, a suitably shaped yoke of additional laminations may be provided to complete the magnetic circuits.